1. Field of the Invention
The invention generally relates to integrated circuit devices utilizing fuse elements and, more particularly, to techniques for reading out the resistance of such fuse elements after a blow process.
2. Description of the Related Art
Fuses are used in integrated circuit (IC) devices for a variety of reasons. As an example, in dynamic random access memory (DRAM) devices, fuses are often used to select redundant circuits to replace failing memory elements, as well as trim various timing and voltage generations circuits. As used herein, the term fuse generally refers to an element whose associated resistance is changed significantly after the element is physically altered (hereinafter “blown”), for example by “cutting” the element with a laser or applying a predetermined programming or “blow” voltage across the element.
Fuses whose resistance is decreased after being blown are generally referred to as antifuses. One type of antifuse is a gate oxide (GOX) antifuse, which has a reduced resistance after being blown via application of a blow voltage. Sensing circuits are typically provided on devices utilizing fuses that determine the state of the fuses (i.e., blown or not-blown) and store a corresponding logic value in a set of fuse latches that control device operation.
In certain circumstances, possibly caused by changes in the element structure or programming voltage due to manufacturing process variations, the decrease in resistance of an antifuse may not be sufficient for this sensing circuitry to determine the antifuse has been blown. Therefore, it is often desirable to read out the resistance of one or more fuses, by a test device, after a blow process to assure the resistance of fuses that were blown has decreased adequately to assure the sensing circuits will detect the fuses as being blown.
Conventional read out circuitry typically only provides a binary (e.g., “blown” or “not-blown”) indication utilizing the fuse latch circuitry. Unfortunately, a detailed characterization of the fuse resistances is not possible from this binary indication. As a result, antifuses with marginal decreases in resistance may be indicated as blown during test read-out. However, in the field, changes in operating conditions may cause these fuses to be detected as not blown by the fuse latch circuits, which may cause erroneous device operation and possibly catastrophic failure.
Accordingly, what is needed is an improved technique for reading out antifuse states, preferably that allows for detailed characterization of antifuse resistances after a blow process.